Mass spectrometry is a useful method for identifying a molecule or ion by its mass-to-charge ratio (m/z). Mass spectrometry has been applied to the study of proteins, organelles, and cells to characterize molecular weight, products of protein digestion, proteomic analysis, metabolomics, and peptide sequencing, among other things. A limitation of mass spectrometry is the difficulty in rapidly measuring biomolecules or macromolecules of high mass-to-charge ratio.
Recent progress in mass spectrometry for biomolecules includes electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI). An ESI source can extend the observable mass range by creating ions from large molecules without fragmenting them. However, ESI may produce a number of charge states or multiply-charged ions that often leads to unnecessarily complex mass spectra. Moreover, the signal of a particular biomolecule may be distributed over many peaks in the mass spectrum which reduces the sensitivity of detection. In general, ESI is not suitable for samples having large numbers of compounds. In some cases, a pre-separation device such as HPLC can be used with an ESI source when the sample contains many compounds. For ion trap mass spectrometry, the multiply-charged ions produced by ESI can cause undesirable space-charge effects inside the ion trap. In contrast, MALDI produces singly-charged ions and can reduce or eliminate the disadvantages of ESI. MALDI is convenient for sample preparation and obtaining the entire mass profile of a complex sample.
For proteomics a mass spectrometer should be able to detect a broad mass range. A high linear dynamic voltage range is essential to this goal. Ion trapping methods such as two-dimensional linear ion traps (LIT) have been useful for proteomics in general by mass-selective ejection of ions from the trap. An advantage of the linear ion trap is that it has a large capacity for ions. This advantage may reduce the space charge effect during mass spectral analysis. However, the mass-to-charge ratio detected by voltage scanning linear ion trap mass spectrometry is limited to about 6000, which is below the mass for most proteins.
There is a continuing need for methods for detecting proteins and biomolecules using a mass spectrometer. There is also a need for an apparatus and arrangement for a mass spectrometer that can detect biomolecular ions over a wide mass range. There is a further need for a mass spectrometer apparatus and methods capable of detecting biomolecules rapidly at high resolution for studies in proteomics.